Zircon U-Pb dating of basalt from Zêtang ophiolite in Tibet and its geological implications

The precise dating of ophiolites from the Yarlung Zangbo suture zone can provide an important constraint on the geological evolution of the Neo-Tethys and the uplift of the Tibetan Plateau. Zêtang ophiolite is the largest outcrop distributed in the eastern Yarlung Zangbo suture zone, but so far there has been no consensus concerning its formation time. This paper reports newly obtained SHRIMP U-Pb dating of zircons from the basalt of this ophiolite mélange, i.e., 154.9Ma±2.0 Ma (95% conf., MSWD=0.98). The age could represent the spreading time of the ancient oceanic crust. In comparison with ages of other ophiolites in the Yalung Zangbo suture zone, this age indicates that the formation of ocean crust was earlier in the east than in the west. The Zêtang ophiolite has essentially the same formation age as the tonalities-bearing island arc rock assemblage, indicating the intra-Tethyan subduction. Based on the geochemistry of basalts as well as the characteristics of the setting and the boninite, the authors hold that Zêtang ophiolite was derived from an intra-oceanic subduction.